Various measures for reduction of emissions of carbon dioxide are being adopted in order to deal with atmospheric pollution and global warming. In the automobile industry, the reduction of emissions of carbon dioxide is highly expected in association with the spread of electric vehicles and hybrid electric vehicles. Thus, development of high-performance secondary batteries used as power sources for driving motors for use in these vehicles is being carried out.
In particular, a higher capacity and cycle property are required for the secondary batteries used for driving the motors. In view of this, lithium ion secondary batteries having high theoretical energy are gaining increasing attention among other types of secondary batteries.
The lithium ion secondary batteries are required to store a large amount of electricity in positive electrodes and negative electrodes per unit mass in order to increase energy density in the lithium ion secondary batteries. Thus, it is quite important for the lithium ion secondary batteries to determine appropriate active materials used in the respective electrodes so as to fulfill such a requirement.
Japanese Unexamined Patent Application Publication No. 2004-311429 has proposed an electrode material and an electrode structure capable of improving performance of a lithium ion secondary battery with low resistivity, high charge-discharge efficiency and high capacity, and has further proposed a secondary battery using these electrode material and electrode structure. In particular, the electrode material disclosed in Japanese Unexamined Patent Application Publication No. 2004-311429 includes solid-state alloy particles mainly containing silicon, in which a microcrystalline or amorphous substance containing an element other than silicon is dispersed in microcrystalline silicon or amorphous silicon.
However, the lithium ion secondary battery using the electrode material described in Japanese Unexamined Patent Application Publication No. 2004-311429 has a problem of decreasing cycle life of the electrode because of a shift from an amorphous state to a crystalline state caused when silicon is alloyed with lithium and thereby resulting in a great change in volume. In addition, in the case that the Si series active material is used, a capacity generally has a trade-off relationship with cycle durability. Thus, there has been a demand for development of active materials capable of concurrently ensuring a high capacity and high cycle durability.